1. Field of the Invention
The present invention relates In general to a catalytic converter installed in an exhaust system of a motor vehicle to reduce noxious components contained In a gas exhausted from an engine of the motor vehicle, More specifically, the present invention is concerned with a catalytic converter which has a honeycomb metallic catalyst bed unit mounted therein.
2. Description of the Prior Art
Hitherto, honeycomb beds have been widely used as a catalyst carrier for catalyst converters. Nowadays, some of the honeycomb beds are constructed of a sheet metal, such as a Fe--Cr--Al system ferrite stainless steel sheet metal (viz., 20Cr--5Al--La--Fe) or the like.
To produce such honeycomb metallic catalyst beds, various methods have been proposed and put into practical use. One method is disclosed in Japanese Patent First Provisional Publication 1-242152. In this method, an elongate corrugated sheet metal and an elongate flat sheet metal are put on each other and then they are rolled up to produce a metallic bed in the shape of cylinder or elliptic cylinder. Then, to suppress relative displacement between the two sheet metals, the metallic bed is subjected to a soldering process to solder or braze contacting portions of the two sheet metals. Laser soldering and vacuum brazing methods are commonly used for such soldering. In another method, corrugated sheet metals of different widths and flat sheet metals of different widths are alternately put on one over another to produce a metallic bed in the shape of cylinder or elliptic cylinder. Then, the metallic bed is subjected to a soldering process for soldering the sheet metals. The metallic bed thus produced in the above-mentioned manners has a plurality of through cells 5 as is shown in FIG. 20. In this drawing, denoted by numerals 1 and 3 are the corrugated and flat sheet metals respectively. These two types of sheet metals 1 and 3 are soldered or brazed at portions 7. As shown, each of through cells 5 defined In the metallic bed has a generally triangular cross section. The produced metallic bed is then subjected to a catalyst treating process to have surfaces of the through cells 5 coated with catalyst layer. Usually, for the catalyst treating process, a dipping method is used wherein the metallic bed is dipped in a wash coat solution for a given time. The wash coat solution Is a solution containing catalyst, .gamma.-alumina and additives. Thus, when the metallic bed is taken out from the solution, the entire surfaces of the through cells 5 are coated with the wash coat layer or catalyst layer. The metallic bed is then heated to dry up the catalyst layer on the surfaces. The metallic catalyst bed is then encased in a metallic case and soldered to the same.
However, due to inherent construction, the catalytic converter produced in the above-mentioned conventional methods has the following drawbacks.
First, as is seen from FIG. 20, when the metallic bed is dipped up out of the wash coat solution, surfaces of through cells are coated or wetted with the solution or catalyst layer 9. However, as is seen from this drawing, at portions "T" where an acute angle is defined between mutually soldered segments of the two types of sheet metals 1 and 3, there is collected a larger amount of catalyst due to the nature of surface tension of solution at such acute angle spaces. This means that an effective surface area of the catalyst layer on the surfaces of the through cells 5 is reduced by a certain degree and most of the catalyst in the portions "T" is wastefully used.
Second, usage of two types of sheet metals tends to lower the solidity or mechanical strength possessed by the metallic bed. In fact, the flat sheet metal can not smoothly absorb the thermal stress like in a manner as is exhibited by the corrugated sheet metal. Thus, when the metallic bed constructed of the two types of sheet metals is exposed to the highly heated exhaust gas for a long time, a considerable deformation tends to occur in the bed due to the thermal stress difference between the two types of sheet metals.
Third, soldering or brazing the two types of sheet metals needs a skilled and thus expensive technique. If the soldering or brazing is poorly made, undesired film-out phenomenon of the layered sheet metals of the metallic bed tends to occur.